Ink drop coupling capacitance compensation

ABSTRACT

A character generator is connected to the charging electrode of an ink jet printer nozzle through a compensating circuit which modulates the charging voltage applied to each individual ink drop in accordance with the charges on one or more preceding drops in order to compensate for the capacitance effect of the preceding drops on said one individual drop.

United States Patent [191 Haskell et al.

[4 1 Jan. 29, 1974 [54] INK DROP COUPLING CAPACITANCE 3,512,173 5/1970 Damouth 346/75 COMPENSATION I 3,631,511 12/1971 Keur et a1. 346/75 [75] lnventors: John W. Haskell, Endwell; Sherman Tsao, Apalachin, both of Primary Examiner-Joseph W. Hartary Attorney, Agent, or Firm-Francis V. Giolma et a1. [73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Sept. 21, 1972 [57] ABSTRACT 21 A 1. N 291,124 1 l 1 PP A character generator is connected to the charging electrode of an ink jet printer nozzle through a com- [52] US. Cl. 346/75 pensating circuit which modulates the charging volt- [51] Int. Cl. G01d 18/00 e applied to each individual ink drop in accordance [5 8] Field of Search 346/75 with the charges on one or more preceding drops in order to compensate for the capacitance effect of the [5 6] References Cited preceding drops on said one individual drop.

UNITED STATES PATENTS I 3,476,874 11/1969 Loughren 346/75 X 10 Claims, 21 Drawing Figures M 20 1o ,24 DELAY R STORED SUMMING CHARACTER 14 22 CIRCUIT i GENERATOR 25 2 81 DEI AY M2 L i 1 PATENTED 3.789.422

SHEU 1 0f 4 5 (5 O O O O FIG. I

ROMIE R0 ]8\ sf 2O @2356 [i0 DELAY W31 /24 2 EER EE'EBTER 4 22 I SUMMING E GENERATOR 23 CIRCUIT ib:

DELAY M2 i I L J F 1 FIG. 2

FIG. 4a

fH- w FIG. 4b

vm M FIG. 4d

PATENTED I I 3.789.422

SHEET? 0? 4 INPUT RI OUTPUT I I I FlG.7b- T I j FlG.7c

' PRIOR ART FIG. 70

ARGENTINA ANNIE CLOCK GENERAToR NETWORK GATE DRIVER -15 104 T10 T00" [I08 H06 58 ALIGNMENT SAMPLE A TNAE DELAY GATE H6 8 ILIUULIULILILILLII. FIG. "9d

PL FIG. 9e

- I fl i w g 1 I "50 /I0 I 1 ,54 58 L CHARACTER IfII) DELAY my) DELAY I 5 GENERATOR i CIRCUIT GLRGUIT g I 40 i 56 BA I I I I I FIG.3 I A I Pmmiumz w I 3,789,422

A B 44 (TWO POSITION I SHIFT REG.) 76

FIG. 5

' T0 TRANSDUCER 4e DROPLET GENERATIOU a FREQUENCY SOURCE CHARGE 34 AMPLITUDE X 5 a 90 ,58 CONTROL 94 L OR CED 95% I CHARACTER Z GENERATION 96 SYSTEM ,56 V 80(THREE POSITION SHIFT REG.)

-zm TRANSDUCER 4e ,76

FIG. 6

. I 'DROPLET GENERATION] FREQUENCY SOURCE LEVEL w x Y z w x Y z WHERE w {111)+9,111-51)+ 111-2511} 9 .15 x= 4 {111)+9 111-511 B .05 1 {111)+ 111-51) FIG. 10

FIG. 11 FIG. 12

FIELD OF THE INVENTION The invention relates to ink drop printing and it has reference in particular to the generation of the proper charging voltage such that accurate compensation for the drop coupling capacitance distortion is achieved.

DESCRIPTION OF THE PRIOR ART Richard G. Sweet, in Technical Report No. 1722-1, dated March 1964, of Stanford Electronics Laboratories, Stanford University, prepared under Signal Corps Contracts, DA36 (O39) SC87300 and DA36 (039) AMC-O3761E, entitled High Frequency Oscillography with Electrostatically Deflected Ink Jets, on page which the adverse affects of the charge on a justformed ink drop upon the charge of a following ink drop being formed are compensated for by utilizing an amplifier to amplify a video signal whenever a previous drop was charged.

OBJECTS AND SUMMARY OF THE INVENTION Generally stated, it is an object of the invention to I provide an improved quality of ink drop printing.

More specifically, it is an object of this invention to provide for modifying the charge applied to one ink drop in an ink drop printer in accordance with the value of the charges on the preceding one or more ink drops.

Another object of this invention is to provide for' using storage means to provide for modifying the charge on an ink drop being formed in an ink drop printer in accordance with the charges on one or more ink drops which have just previously been charged.

Yet another object of the invention is to provide for modifying the charge on an ink drop being formed in accordance with a voltage defined by the equation where K is the number"bttiiEsfiihavifig a non-- negligible coupling capacitance'with the drop being formed.

Still another object of the invention is to provide for detecting if one or more previously formed drops ofink in an ink jet printer have been charged and for modifying the charge on a drop currently being formed in accordance with the greater of two or more modifying voltages dependent on the charges on said previously charged drops.

Another important object of the invention is to provide in an ink drop printer for modifying the charge on a drop just being formed so as to substantially neutralize the capacitance effects on said drop of drops previously charged and thereby insure accurate positioning of said drop being formed.

Yet another object of the invention is to provide for connecting a character generator charging electrode signal source to the charging electrode in an ink drop printer through a plurality of circuits each of which apply different functions of the charging electrode signal to the charging electrode.

It is also an object of the invention to provide for connecting a charging signal source to the charging electrode in an ink drop printer by a plurality of delay means having different delay characteristics for modifying the charging signal at any instant in accordance with previous values of thecharging signal.

Still another important object of the invention is to provide in an ink drop printer for utilizing a plurality of delay means having different characteristics to modify the charging signal at the instant of forming one drop in accordance with the charging signals at the instance of forming one or more previously formed drops.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular descriptions of preferred embodiments of the invention, as illustrated in the accompanying drawing.

DESCRIPTION OF THE DRAWING sated ink jet printer charging circuit embodying the invention in one form;

FIG. 3 is a schematic circuit diagram of a compensated charging circuit embodying the invention in a different form;

FIGS. 4a 4d show curves illustrating the voltage relations in FIG. 3;

FIG. 5 is a schematic circuit diagram of a compensated charging circuit diagram embodying the invention in another form;

FIG. 6 is a schematic circuit diagram of a compensated charging circuit embodying the invention in yet another form;

FIG. 7a is a circuit diagram of a R-C compensating network;

FIG. 7b is a waveform of a typical input signal;

FIG. is a waveform of the resulting output signal from the R-C compensating network;

FIG. 8 is a schematic block diagram of a compensating circuit'embodying the R-C network of FIG. 6a;

FIGS. 9a 9d show waveforms for the circuit of FIG.

FIG. 10 is a table of compensated charging amplitude voltage values for first and second order compensation according to the formulas of the invention;

FIG. 11 is a typical uncompensated staircase waveform of a charge amplitude control used with the circuit of FIG. 5 to provide the compensated values of the table in FIG. 9; and

FIG. 12 is a schematic showing of the WXYZ voltage divider of FIG. 6 showing the calculated values of resistance to give the voltage values of the table in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS the Stream 1 and the numerals 3 and,4 designate drops which have previously broken off. The numeral 5 designates a charging electrode for placing a charge on the drops as they are formed for deflecting them in accordance with a predetermined pattern onto a record or the like. The reference numeral 6 designates generally a source of voltage for the charging electrode 5. The following is a generalized derivation for the modified voltage pulse to be applied to a drop being formed in order to accurately position it on the document.

In the following analysis the symbols I ,(t) and q,(t) shall mean the voltage potential and charge respectively at time t on the ith conducting body in FIG. 1. From elementary electrostatics theory, there is a simple expression relating the charges to the voltage potentials which for the configuration of FIG. 1 is:

where the C 's are the coefficients of capacitance and C s, i a j, are the coefficients of electrostatic induction. Alternatively, the above expression may also be rewritten in the form The C u S are called direct capacitances and in particular, Ci 's are self-capacitances. it is known also from electrostatics theory that C 2 O, Q; s O for i 9* j, and C=U 2 I Without loss ofgenerality, D and I (t) may be set to zero or at ground level since Equation (2) clearly implies that the charges only depend on the relative voltage potentials of the conductors. With I ,(t) (t) 0, Equation (1) simplifies to (1 (l)=2C q (t); i=1,2, .,5 Conductor 5 in FlG. 1 represents the charging electrode hence where f(t) is a voltage pulse proportional to a curve which traces through the sequence of intended droplet deflections. Combining Equation (3) into Equation (5) and solve for 0) and I explicitly Now substitute the above values for D 0) and D 0) into the expression for q (t) in Equation (3) q (t) u s) 2-5 40) zsfl') 4 ri e The implication is that if f( t) is in some proportion to the intended deflection of the droplet presently being formed, then its resulting charge would not be proportional to the deflection as intended.

Considering the fact thatthe sequence of droplets are formed at apart in time, the phenomenon described by Equation (9) maybe written as a second order difference equation 1 where (II) and coefficients D f, and D corresponds to the obvious terms in Equation (9). From Equation (10),

. (13) Substituting Equations (12) and ('13) into Equation (10) gives (1 From physical arguments one can conclude for the configuration of FIG. 1 that 1 D, D in fact, one estimate of a particular configuration which also has been substantiated be experiments puts the values of D and D at 0.15 and 0.05, respectively. Thus, some terms in Equation (14) clearly have only a minor influence on the value of q" From engineering point of view it is desired that 11"" be proportional to f lAn obvious solution is to modify the charging voltage before it is applied to the charging electrode. The form of the charge distortion in Equation (14) immediately suggests the following correction V(z) =f(r) D n-m m n-2m) As a verification, consider the substitution of V(t) for The difference between q" and DJ" is a measure of the charge distortion which according to Equation (16) is (I In view of the smallness of the numerical values of D and D one concludes simply that |q -D fl" 0 very rapidly as n increases. That is, one essentially has the desired result of achieving droplet charges in direct proportion to the intended droplet deflections. This is achieved by using the modified charge voltage pulsel/(t) in place of f (t).

An immediate generalization of the above analysis is- V0 =0: ar-tat (18) where a is an overall amplitude factor; K is the number of preceding droplets having non-negligible coupling capacitance with the dropletjust being formed; B 's are constants depending on the coupling capacitance values. The case with K 2 and aB =l reduces to that which was considered in detail. Also the values of the parameters [3, may be determined by electrically tuning or adjusting the resistors in the circuit of FIG. 2 for the best droplet deflections.

Referring to FIG. 2, the reference numeral denotes a stored character generator circuit for applying a deflection signal to a Charging Electrode 5. In order to accurately position the drops a Compensating Circuit 14 is connected between the stored Character Generator 10 and the Charging Electrodes 5. The compensation circuit 14 may comprise a Resistor 16 connecting the stored Character Generator 10 to the Charging Electrode 5 for applying a voltage to the Charging Electrode 5 without time delay in accordance with the actual signal from the stored Character Generator 10. In addition to the Resistor l6, compensation storage means such as a shift register or a Delay Circuit 18 may be connected in parallel with the Resistor 16. The Delay Circuit 18 has a delay equal to 0-1 which is the time interval between the drops. Connected in series with the Delay Circuit 18 is a Resistor 20 having a value equal to R /B, where R is the value of Resistor 16. This provides a compensating voltage which is dependent on the charging voltage applied to the previous drop which occurred 0-! ahead of the drop just being formed. Additional compensation may be supplied by one or more additional delay circuits such as the Delay Circuit 22 having a delay equal to 20'! and which is connected to the charging electrode 5 through a resistor having a value Ru/Bg- This circuit provides compensation in accordance with the second drop ahead of the drop currently being formed. The Resistor l6, Delay Circuit 18 and Delay Circuit 22 may be connected to a Summing Circuit 24 for applying the resulting voltage to the Charging Electrode 5 through an operational 6 Amplifier 26 having a feedback circuit comprising a Resistor 28 having a value equal to ax R Referring to FIG. 3, a different embodiment is shown of a compensating circuit embodying the invention. As shown, a Compensating Circuit 30 connects the Character Generator 10 to the Charging Electrode 5. The Compensating Circuit 30 comprises Delay Circuits 32 and 34, each having a delay equal to at and connected in series circuit relation. An adjustable Rheostat 36 connects a point between the two delay circuits to the Charging Electrode 5 through an Amplifier and Adder Circuit 38. A second Rheostat 40 connects the Delay Circuit 34 to the charging electrode through the Amplifier and Adder 38. Additional delay circuit may be added on when needed. A Conductor 42 provides a direct connection from the character generator to the Charging Electrode 5 through the Amplifier and Adder 38. Accordingly, each time a signal is applied from the Character Generator 10 to the Charging Electrode 5, at the same time delayed signals will be applied to the Charging Electrode 5 from the Delay Circuits 32 and 34, which are representative of the charges on previously charged drops which were generated (rt and 20! ahead of the drop currently being charged. The curves in FIGS. 40 through 4d show the resulting compensated charging signal in FlG. 4d generated from an original charging signal shown in FIG. 4a.

Referring to FlG. 5, the reference numeral 44 designates a nozzle having a Transducer 46 for vibrating the nozzle to cause a stream of Ink 1 issuing from the nozzle tobreak up into dropswhich are charged by a Charging Electrode 5 and deflected by Deflection Electrodes 48 either into a Gutter 50 or onto a Document 52 for printing thereon. Instead of applying signals from a Charge Amplitude Control Circuit 54, which may, for example, produce a staircase type signal, as shown, to the Charging Electrode 5 in conjunction with character generating signals from a Character Generation System 56, which provides a pulse output characteristic of predetermined characters, the Charge Amplitude Control Circuit 54 may be connected to a Charging Electrode Driver 58 through AND gate 60 and a maximum priority OR circuit 62. A Voltage Divider 64 comprising Resistors 66 and 68 is connected between the Charge Amplitude Control Circuit 54 and ground at a point X. Point Y between the Resistors 66 and 68 is connected to the OR circuit 62 through AND gate 72. A two-position Shift Register 74 is connected to the Character Generator Circuit 56 and is connected to be advanced by a Droplet Generation Frequency Source 76, which is alsovconnected to the Transducer 46 for producing the ink drops. Stage A of the Shift Register 74 is connected to the AND 72, aswell as the AND 60, while Stage B is connected to the AND 60 only. The Resistors 66 and 68 are so proportioned that the output at X equals f(t)+fi,f( t0r) and the output Y equals f(t). The output Y corresponds to the signal normally applied to the charging electrode 5 as an uncompensated signal, while the signal at X corresponds to the signal to be applied to the Charging Electrode 5 compensated in accordance with the charge on a previous drop, which can be readily determine with a uniform staircase signal of the type being used. Accordingly, the charge on each drop being formed is compensated in accordance with the value of the charge on a previous drop, if the previous drop was charged, The Shift Register 74 determines whether or not the previcompensated signal voltage will be gated through the AND 72 and OR 62 for application to the Charging Electrode 5. j I

Referring to FIG. 6, 'it will be seen that a 3-p0sition Shift Register 80 is utilized in conjunction with AND circuits 82, 84, 86, 88 and a maximum priority OR circuit 90 for applying compensated charging signals to a Charging Electrode 5 through a Driver 58 under the control of a Character Generation Circuit 56 and a Droplet Generation Frequency Source 76. The Charge Amplitude Control Circuit 54 is connected directly to the AND 82 and to a Voltage Divider 92 comprising Resistors 93, 94, 95 and 96 providing voltages at the WXYZ points in accordance. with the equations Z =f( The 3-position Shift Register 80 gates signals from the W, X, Y and Z points of the Voltage Divider 92 through AND circuits 82, 84, 86 and 88, depending on whether the first or second previous drop was charged or not.

Referring to FIG. 7a, a Compensating Network 100 is shown similar to that described in the article by R. G. Sweet, hereinbefore referred to. As shown, the network comprises a capacitor C connected with a resistor R2 in a T network having a resistor R connected between the input and output terminals of one leg. This network, when provided with input waveform, such as shown in FIG. 7b modifies the waveform to provide the output shown in FIG. 70. The disadvantage of such a network is that exact compensation occurs only at one instant of the clock cycle, whereas the droplet may separate at any time during this cycle.

In order to provide an improved compensating circuit, the arrangement, as shown in FIG. 8, may be used. As shown, a typical Compensating Network 100 is connected between a Source 102 of clock signals and a non-sequential Scan Character Generation Circuit 104 and a Charging Electrode Driver 58 for applying a charging signal to a Charging Electrode 5. A Sample and Hold circuit 106 is connected between the Sweet Network 100 and the Charging Electrode Driver 58. This circuit is controlled by a Sample Gate 108 and a Delay Circuit 110 so that the Sweet Network 100 output is gated to the Charging Electrode 5 at the exact instant of correct compensation. Thus, the correct charge amplitude is presented to the droplet regardless of when it separates during the cycle. The Delay Circuit 110 is provided to insure the Sample Gate 108 occurs at the correct time relative to the Character Generator output. Timing relations are shown by the typical waveforms of FIGS. 9a through 9e.

FIG. 10 shows a table of charging voltage values obtained at the junctions W, X, Y and Z in the circuit of FIG. 6, when utilizing an uncompensated staircase signal such as shown in FIG. 11. The values of the Resistors 93 through 96 are shown in'FIG. 12 for obtaining the values in the table of FIG. 10. A typical value for R is 1000 ohms.

- From the above description and the accompanying drawing it will be apparent that we have provided in a simple and effective manner for accurately compensating the charging electrode voltage in an ink jet printer for interdrop capacitance effects. By utilizing storage means such as the delay devices for providing compensation to the charging electrode voltage for a drop currently being formed in accordance with values of the voltages applied to previous drops, greatly improved print quality is obtainable. The delay circuit technique is applicable to non-uniform voltage signals as well as that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In an ink jet printer wherein a stream of ink drops.

is directed from a nozzle toward a recording medium,

electrode means adjacent said-nozzle and the path of said stream,

circuit means connecting video means to said electrode means to apply signals thereto to selectivelycharge different ones of said ink drops to deflect said ink drops in accordance with predetermined video signals to position selected ones of said drops on said recording medium in a predetermined pattern, and

compensation means including storage means connected between said circuit means and said electrode means for detecting the charges on previous drops and modifying the video signal from said circuit means for charging a'particular drop in accordance with the equation V(!) oz{f(r) ,B,f(trrr) 2 o w here V6) is the compensated charging voltage function; at is the drop generation period; and a is a scaling or amplitude constant; to minimize the effects of said previous drop charges on said particular drop.

2. The invention as defined in claim 1 characterized by said storage means comprising a delaynetwork.

3. The invention as defined in claim 1 characterized by said storage means comprising a plurality of delay networks connected to said charging electrode through asumming network.

4. The invention as defined in claim 3 characterized by said delay networks having different values which are integral multiples and being connected in parallel circuit relation between said circuit means and said charging electrode.

5. The invention as defined in claim 3 characterized by said delay networks comprising similar elements connected in cascade between said circuit means and said charging electrode.

6. The invention as defined in claim 1 characterized by said compensation means including a voltage divider having sections proportioned according to the terms of t e sq a ter M f M 32]"(1 2oz) abuses tea betweeii'saicl circuit means and ground, and gate means controlled in accordance with the charge condition of previous drops connecting different sections of said voltage divider to, said charging electrode.

7. The invention as defined in claim 6 characterized by a shift register connected to respond in accordance with said video signals and connected to said gate means to control said gate means in response to charge condition of said preceding drops.

8. The invention as defined in claim 7 characterized by said gate means being connected to said charging electrode by a maximum priority gate means which passes only the maximum of a plurality of signals.

9. The invention as defined in claim 8 characterized by said shift register being advanced in accordance with the frequency of formation of said ink drops.

10. In an ink jet printer wherein a stream ofink drops is directed from a nozzle toward a recording medium,

electrode means adjacent said nozzle and the path of said stream,

circuit means connecting video means to said electrode means to apply signals thereto to selectively charge different. ones of said ink drops to deflect said ink drops in accordance with predetermined video signals to position selected ones of said drops on said recording medium in a predetermined pattern, and

compensation means including a RC network and a ticular drop. 

1. In an ink jet printer wherein a stream of ink drops is directed from a nozzle toward a recording medium, electrode means adjacent said nozzle and the path of said stream, circuit means connecting video means to said electrode means to apply signals thereto to selectively charge different ones of said ink drops to deflect said ink drops in accordance with predetermined video signals to position selected ones of said drops on said recording medium in a predetermined pattern, and compensation means including storage means connected between said circuit means and said electrode means for detecting the charges on previous drops and modifying the video signal from said circuit means for charging a particular drop in accordance with the equation V(t) Alpha (f(t) + Beta 1f(t- sigma t) + Beta 2f(t-2 sigma )-) where V(t) is the compensated charging voltage function; sigma t is the drop generation period; and Alpha is a scaling or amplitude constant; to minimize the effects of said previous drop charges on said particular drop.
 2. The invention as defined in claim 1 characterized by said storage means comprising a delay network.
 3. The invention as defined in claim 1 characterized by said storage means comprising a plurality of delay networks connected to said charging electrode through a summing network.
 4. The invention as defined in claim 3 characterized by said delay networks having different values which are integral multiples and being connected in parallel circuit relation between said circuit means and said charging electrode.
 5. The invention as defined in claim 3 characterized by said delay networks comprising similar elements connected in cascade between said circuit means and said charging electrode.
 6. The invention as defined in claim 1 characterized by said compensation means including a voltage divider having sections proportioned according to the terms of the equation V(t) f(t) + Beta 1f(t- sigma t) + Beta 2f(t-2 sigma t)-connected between said circuit means and groUnd, and gate means controlled in accordance with the charge condition of previous drops connecting different sections of said voltage divider to said charging electrode.
 7. The invention as defined in claim 6 characterized by a shift register connected to respond in accordance with said video signals and connected to said gate means to control said gate means in response to charge condition of said preceding drops.
 8. The invention as defined in claim 7 characterized by said gate means being connected to said charging electrode by a maximum priority gate means which passes only the maximum of a plurality of signals.
 9. The invention as defined in claim 8 characterized by said shift register being advanced in accordance with the frequency of formation of said ink drops.
 10. In an ink jet printer wherein a stream of ink drops is directed from a nozzle toward a recording medium, electrode means adjacent said nozzle and the path of said stream, circuit means connecting video means to said electrode means to apply signals thereto to selectively charge different ones of said ink drops to deflect said ink drops in accordance with predetermined video signals to position selected ones of said drops on said recording medium in a predetermined pattern, and compensation means including a RC network and a sample and hold gate with circuit means for delaying the gating operation thereof connected between said circuit means and said electrode means for detecting the charge on a previous drop and modifying the video signal from said circuit means for charging a particular drop in accordance with the value of said previous drop charge to minimize the effects of said previous drop charge on said particular drop. 